The present invention relates to a conditioning method of gas turbine engine components for aerodynamic noise reduction, more particularly a method of reducing the surface roughness of blades and vanes of a turbine engine compressor to a level that allows for a change in air flow characteristics and creates aerodynamic noise reduction.
Gas turbine engines are not only used as aircraft engines but also as aero derivative engines. For example industrial engines extensively used in industrial service, marine and land transportation. Aero and ground-based engines are built with many similar components.
One of the sections of a gas turbine engines is a compressor. The compressor delivers compressed air to a combustion chamber. Fuel mixed with compressed air is burned and transformed into a massive volume of gasses. These gasses expand and flow through the stator and rotor blades of the turbine converting produced energy to its kinetic equivalent, thrust or rotation.
Higher compressor pressures and temperatures can allow for greater efficiencies unfortunately with a higher noise level. The pressures and temperatures are limited by the characteristics of materials. Noise level is dependent on the quality of the flow through the gas path.
Among others the aerodynamic performance relies on surface roughness of the compressor elements. Especially, but not limited to blades and vanes, surface roughness can be the result of deposition, erosion, or surface finish and causes friction and profile losses.
Pertinence of the surface roughness for compressor turbine blade and vane airfoils has been proven through extensive research and the results are published, for example in Kind, R. J., Serjak, P. J., and Abbott, M. W. P., 1998, “Measurements and Prediction of the Effects of Surface Roughness on Profile Losses and Deviation in a Turbine Cascade”, ASME Journal of Turbomachinery, Vol. 120, pp. 20-27.
Generally, flow in the compressor is inherently unstable, turbulent and fully three-dimensional regardless of the inlet flow type: subsonic, supersonic or in some cases transonic. Depending on the blade design along the curved profile the flow may be laminar, transitional or turbulent; the flow characterizes the boundary layers. Quality of the flow is more important on the suction, lower pressure side of the airfoil.
Laminar flow formed near the leading edge defines the laminar boundary layer. Influence of the surface roughness on this layer is limited.
Turbulent flow which may be of low, medium and high intensity, defines the turbulent boundary layer. Presence of turbulence is required to create higher pressure, compared to an identical laminar flow system.
The transition flow is located between the two aforementioned types of flow; it defines the transition boundary layer. In this region the separation bubble appears and reattaches.
A lot of controversy surrounded the effect of turbulence in the early days. It concerned the creation of a separation bubble, its reattachment and effect on turbulence creation. Regardless of the controversies, the latest results proved the existence of a reduced loss level with lower surface roughness.
The aerodynamic noise reduction credit from improved surface smoothness depends on conditions in the regions where the airfoils have to work; it is especially derived from Mach and Reynolds numbers based on inlet velocity, the blade chord and the kinematic viscosity.
Compressor fouling obtained from deposition of foreign particles can induce separation of the boundary layer that results in unexpected pressures on the blade and vane airfoil surfaces and thus produce, according to Zwebek, A. I., 2002, “Combined Cycle Performance Deterioration Analysis” (PhD Thesis), Cranfield, vibration and noise. This problem is seldom improved during regularly scheduled maintenance intervals.
Engine washing is used to reduce unwanted residuals, a number of the techniques are presented in U.S. Pat. Nos. 5,868,860; 6,394,108 and 8,479,754.
Noise reduction methods not involving blade and vane airfoils surface conditioning are published in U.S. Pat. Nos. 3,572,960 and in 4,199,295.
Deterioration associated with profile losses of blade and vane airfoil resultant from impact and erosion damage is postponed by applying a hard coating using techniques like: physical vapor deposition (PVD), chemical vapor deposition (CVD) and high velocity oxygen fuel thermal spraying (HVOF) described in U.S. Pat. No. 8,118,561. Unfortunately these kinds of coatings are not the best solution for aerodynamic noise reduction. Recommended coating thickness of 25 μm results in a 4% loss of pressure ratio as pointed out in Suder, K. L., Chima, R. V., Strazisar, A. J. and Roberts, W. B., 1994, “The Effect of Adding Roughness and Thickness to a Transonic Axial Compressor Rotor”, prepared for the 39th international Gas Turbine and Aeroengine Congress and Exposition sponsored by the American Society of Mechanical Engineers, The Hague, Netherlands, Jun. 13-16, 1994.
Surface roughness profile (Ra) of blades and vanes achieved during the manufacturing process such as casting, forging or machining varies between 10 and 0.8 μm. Surface properties improvement process such as peening applied after these processes may smooth the surface down to Ra 0.4 μm as disclosed in U.S. Pat. No. 4,454,740. Hand polishing, tumbling with abrasive media or electrochemical machining (ECM) may lower Ra down to 0.2 μm. ECM process presented in U.S. Pat. No. 8,764,515 reaches 0.025 μm. Unfortunately ECM is not environmentally friendly. Electrolytes used in this process are mixtures of hydrofluoric acid (HF), hydrochloric acid (HCl) and sulfuric acid (H2SO4). The mixture also may cause corrosion.
Processes like polishing, tumbling, vibro-lapping and electro-polishing used for surface improvement of hard, environmental barrier coated components are demonstrated in U.S. Pat. No. 6,576,067. This fabrication method allows achieving a surface roughness (Ra) of 3 μm. Another method, exploring slurry application in U.S. Pat. No. 8,673,400 permits a decrease only to 40 μm.
Other present advanced machining and finishing processes, for instance electron-beam or laser machining, are not able to accomplish better results. Of course high vacuum plasma etching technology offers some solutions like arc cathode smoothing presented in U.S. Pat. No. 6,517,688 where surface finish (Ra) of 1.5 μm was obtained. Alternatively, ion beam polishing in U.S. Pat. No. 5,529,671 where only 20 nm is removed from the substrate. A smoothing example of low initial surface roughness (Ra), 10 nm, is U.S. Pat. No. 6,375,790 where a gas cluster ion beam is used for microelectronic materials. In this field and as well in conventional, micro and nano optics, magnetic storage technology, semiconductor technology and analytical techniques plasma etching is used wisely. A good illustration of those applications is publication of Frost, F., Fechner, R., Ziberi, B., Völlner, D., Flamm, D. and Schindler, A., 2009, “Large area smoothing of surfaces by ion bombardment: fundamentals and applications”, Journal of Physics: Condensed Matter, 21.
What is needed for the application presented above is a method which can harness high quality surface finishing technology such as plasma etching and transform it into a technology which addresses the flow issue by enhancing blade and vane airfoil smoothness which in turn reduces aerodynamic noise.